Traditionally, the efficacy of many cancer therapies was believed to arise from the cytotoxicity derived from chemotherapy- or radiation-induced DNA damage. Such DNA damage was considered to trigger an apoptotic response. See Eastman et al., Cancer Invest., 10: 229-240 (1992); Allan, D. J., Int. J. Radiat. Biol., 62: 145-152 (1992). Apoptosis is conceptualized as an inducible preprogrammed pathway of sequential biochemical events, leading to activation of calcium- and magnesium-dependent endonucleases that cleave the nuclear chromatin at selective internucleosomal linker sites. Signals generated at the membrane of the affected cell activate neighboring cells and infiltrating macrophages to phagocytize the dying cell and its disintegrating nucleus.
An early hypothesis on the nature of the lethal damage produced by ionizing radiation identified heterologous double strand breaks in the DNA as the most common type of lesions that lead to mammalian cell death. See Radford, I. R., Int. J. Radiat. Biol., 49: 611-620 (1986); Ward, J. F., Prog. Nucleic Acid Mol. Biol., 35: 95-125 (1988). Such lesions are produced in the DNA by direct interaction with X-rays, or with reactive oxygen intermediates generated within the cell by the radiation. See Steel et al., Int. J. Radiat. Biol., 56: 525-537 (1989). While mammalian cells are proficient in repairing most DNA double strand breaks, not all such lesions are repairable. See Ward, J. F., Prog. Nucleic Acid Mol. Biol., 35: 95-125 (1988). Residual unrepaired DNA lesions can lead to post-mitotic cell death. See Bedford, J. S., Int. J. Radiat. Oncol. Biol. Phys., 21: 1457-1469 (1991). Therefore, until recently, inefficiency of DNA repair was thought to play a key role in radiation sensitivity.
Similarly, some chemotherapies, for example anthracycline daunorubicin (DNR), were believed to induce cytotoxicity as a result of drug-induced damage to DNA. It was suggested that damage to genetic material could result from free radicals stemming from the quinone-generated redox activity, from intercalation-induced distortion of the double helix, or from stabilization of the cleavable complexes formed between DNA and topoisomerase II. See Chabner et al., Cancer: Principles and Practice of Oncology, J.B. Lippencott Co., Philadelphia, Pa. Pp 349-395 (1989). However, the mechanism by which such damage induced the apoptotic pathway remained unclear.
In recent years, an alternative to the hypothesis that direct DNA damage from cancer therapies mediates induced apoptosis has been established. The sphingomyelin signal transduction pathway for induction of apoptosis has emerged as a leading mechanism in many cancer therapies, including ionizing radiation, tumor necrosis factor α (TNF-α) and daunorubicin. See Haimovitz-Friedman et al., J. Exp. Med., 180: 525-535 (1994); Kolesnick et al., Cell, 77: 325-328 (1994); Jaffrezou et al., Embo J., 15: 2417-2424 (1996); Bose et al., Cell, 82: 405-414 (1995).
Sphingomyelin is a class of sphingolipids, which constitute a major lipid class in the cell, especially the plasma membrane. See Merrill et al., Toxicol. Appl. Pharmcol., 142: 208-225 (1997). Sphingomyelin is compartmentalized into two distinct pools in the plasma membrane. See Linardic et al., J. Biol. Chem., 269: 23530-23537 (1994). It has been proposed that the sphingomyelin pool localized to the inner leaflet of the plasma membrane is dedicated exclusively to intracellular signaling. The observation that there is no difference in sphingomyelin molecular species between the two pools of sphingomyelin in the plasma membrane suggests the importance of compartmentalization in signal transduction. See Fritzgerald et al., Lipids, 30: 805-809 (1995).
Many cancer therapies initiate the sphingomyelin pathway by inducing the rapid hydrolysis of sphingomyelin to ceramide. Ceramide plays a pivotal role in a variety of cellular processes, including regulating programmed cell death. See Merrill et al., Toxicol. Appl. Pharmcol., 142: 208-225 (1997). The specificity of ceramide as a second messenger for apoptosis was demonstrated by the fact that cell-permeable ceramide analogs, but not analogs of other lipid second messengers, were able to recapitulate the effects of TNF-α, Fas, and ionizing radiation and induce apoptosis directly. Induction of apoptosis by ceramide is also stereospecific, since dihydroceramide fails to induce apoptosis. It has been proposed that ceramide initiates apoptosis by activating the stress-activated protein kinase pathway. See Verheij et al., Nature, 380: 75-79 (1996).
While many therapies are successful in initiating the sphingomyelin transduction pathway, the induced apoptotic response may be limited or short-lived. For unknown reasons, tumor cells have abnormal lipid composition, including sphingomyelin. Tumor tissues typically have higher concentrations of sphingomyelin than normal tissues; however, it is possible that some tumor cells have reduced sphingomyelin synthesis capabilities. See Koizumi et al., Biochim. Biophys. Acta., 649: 393-403 (1991); Van Blitterswijk et al., Biochim. Biophys. Acta., 778: 521-529 (1984). Additionally, altered lipid metabolism in tumor cells can result in changes in the intracellular distribution of sphingomyelin. Such redistribution within the plasma membrane can lead to misdirected sphingomyelin which is unable to be acted upon by the sphingomyelin hydrolyzing enzymes responsible for generating ceramide in response to cytotoxic treatment. See Bettaieb et al., Blood, 88: 1465-1472 (1996). Consequently, sphingomyelin re-organization within the plasma membrane can impair a tumor cell's ability to generate ceramide-induced apoptosis and lead to reduced sensitivity to certain therapies.
A need, therefore, continues to exist for a method for overcoming tumor cell alteration of lipid metabolism in order to maximize a tumor therapy utilizing the sphingomyelin pathway for induction of apoptosis.